The present invention relates generally to bus communications, and more particularly to a system wherein functional units coupled to a bus are able to determine their relative positions on the bus.
A bus system provides a structure and a set of communication paths over which a plurality of functional units may send signals to one another. Typically, the signal lines include a common set of signal lines to which the functional units are connected in parallel. The bus system may also include a number of private lines that provide communication paths between particular functional units.
The signal lines are often implemented as generally parallel traces on a printed circuit board, known as a backplane. In this configuration, the functional units are built on circuit boards, which plug into connectors (slots) that extend across the backplane traces and hold the boards perpendicular to the backplane.
Each functional unit on the bus must have a unique address, which should depend only on the physical slot into which the board is placed. For consistency, the units will be numbered from 1 to N with unit 1 being leftmost. Movement to the right will be considered downstream, to the left upstream.
One way of providing slot numbers is to provide a special set of lines, and have each slot connect with only certain of the lines to define a binary code that is representative of that slot's position along the bus. In general, if there are N slots, there should be log.sub.2 N (rounded up to the next highest whole number) lines. Thus in a system having up to 16 functional units, four lines would be dedicated to providing each slot with its unique address. However, backplane traces represent a scarce resource and this technique can be wasteful. Moreover, this technique does not provide a functional unit with the information relating to the total number of units in the system (i.e., the highest address).
An alternative to having dedicated backplane traces defining the bus slot addresses is to provide each unit with a set of user settable switches. At the time of connecting the units to the backplane, the switches are set to a configuration that defines the appropriate binary address. Clearly, the system is prone to human error. Moreover, the system provides no information as to the total number of functional units in the system.